tools: add example json-seq render based on tqdm

Add an example render to test/demo how the json-seq based progress
works. It needs the python `tqdm` package for the actual rendering.

See the output with:
```
$ sudo OSBUILD_TEST_STORE=/var/tmp/osbuild-test-store  \
    python3 -m osbuild --libdir=. --monitor=JSONSeqMonitor --export image \
      --output-dir=/tmp/output-dir ./test/data/manifests/fedora-boot.json | ./tools/osbuild-json-seq-progress-example-renderer
```

tools/osbuild-json-seq-progress-example-renderer

@@ -0,0 +1,91 @@
+#!/usr/bin/python3
+#
+# example how to use the json-seq rendering
+#
+
+import json
+import sys
+
+import tqdm
+
+
+class TqdmProgressRenderer:
+    BAR_FMT = "{desc} ({n_fmt}/{total_fmt}): {percentage:3.0f}%|{bar}|{elapsed}"
+
+    def __init__(self, inf, outf):
+        self._pbar = None
+        self._sub_pbar = None
+        self._inf = inf
+        self._outf = outf
+        self._last_done = 0
+        self._last_sub_done = 0
+
+    def _read_json_seq_rec(self):
+        # *sigh* we really should be using a proper json-seq reader here
+        while True:
+            line = self._inf.readline()
+            if not line:
+                return None
+            try:
+                payload = json.loads(line.strip("\x1e"))
+            except json.JSONDecodeError:
+                self.warn(f"WARN: invalid json: {line}")
+                continue
+            return payload
+
+    def warn(self, warn):
+        if self._pbar:
+            self._pbar.write(warn)
+        else:
+            print(warn, file=self._outf)
+
+    def _init_pbar(self, pbar_name, total, pos):
+        pbar = getattr(self, pbar_name, None)
+        if pbar is not None:
+            return
+        pbar = tqdm.tqdm(total=total, position=pos, bar_format=self.BAR_FMT)
+        setattr(self, pbar_name, pbar)
+
+    def render(self):
+        while True:
+            js = self._read_json_seq_rec()
+            if js is None:
+                return
+
+            # main progress
+            main_progress = js.get("progress", {})
+            total = main_progress.get("total", 0)
+            self._init_pbar("_pbar", total, pos=0)
+
+            ctx = js["context"]
+            pipeline_name = ctx.get("pipeline", {}).get("name")
+            if pipeline_name:
+                self._pbar.set_description(f"Pipeline {pipeline_name}")
+            done = main_progress.get("done", 0)
+            if self._last_done < done:
+                self._pbar.update()
+                self._last_done = done
+                # reset sub-progress
+                self._last_sub_done = 0
+                self._sub_pbar = None
+
+            # sub progress
+            sub_progress = main_progress.get("progress")
+            if sub_progress:
+                total = sub_progress.get("total")
+                self._init_pbar("_sub_pbar", total, pos=1)
+                stage_name = ctx.get("pipeline", {}).get("stage", {}).get("name")
+                if stage_name:
+                    self._sub_pbar.set_description(f"Stage {stage_name}")
+                sub_done = sub_progress.get("done", 0)
+                if self._last_sub_done < sub_done:
+                    self._sub_pbar.update()
+                    self._last_sub_done = sub_done
+
+            # (naively) handle messages (could decorate with origin)
+            self._pbar.write(js.get("message", "").strip())
+
+
+if __name__ == "__main__":
+    prg = TqdmProgressRenderer(sys.stdin, sys.stdout)
+    prg.render()